In golf balls commercially selling, there are solid golf balls such as two-piece golf ball, three-piece golf ball and the like, and thread wound golf balls. Recently, the two-piece golf ball and three-piece golf ball, of which flight distance can be improved while maintaining soft and good shot feel at the time of hitting as good as the conventional thread wound golf ball, generally occupy the greater part of the golf ball market. Multi-piece golf balls represented by the three-piece golf ball have good shot feel while maintaining excellent flight performance, because they can vary hardness distribution, when compared with the two-piece golf ball.
The three-piece solid golf balls are obtained by inserting an intermediate layer between the core and the cover layer constituting the two-piece solid golf ball, and have been described in Japanese Patent Kokai Publication Nos. 174728/1998, 70409/2000 and the like. In the golf balls, it has been attempted to compromise the balance of flight performance and shot feel at the time of hitting by using thermoplastic resin, such as ionomer resin, thermoplastic elastomer, functional group modified thermoplastic elastomer or mixtures thereof for the intermediate layer, to adjust a hardness, hardness distribution and the like of the core, intermediate layer and cover to proper ranges.
In Japanese Patent Kokai publication No. 174728/1998, a multi-piece solid golf ball comprising a core, at least one layer of an intermediate layer and a cover, of which at least one layer of the intermediate layer is formed from as a main component heated mixture of at least two components selected from three components consisting of ionomer resin, thermoplastic elastomer having a terminal OH group, and styrene-butadiene-styrene block copolymer or styrene-isoprene-styrene block copolymer having an epoxy group, and the intermediate layer has a hardness in JIS-C hardness of 40 to 80, and a hardness difference from the center to the surface of the core is not more than 15, is disclosed.
In Japanese Patent Kokai publication No. 70409/2000, a multi-piece solid golf ball obtained by inserting at least one layer of an intermediate layer between a solid core and at least one layer of a cover, of which the intermediate layer is formed from as a main component thermoplastic resin having a Shore D hardness of 8 to 35 and the cover is formed from a cover material comprising thermoplastic resin as a main component and inorganic filler, and the Shore D hardness of the cover is higher than that of the intermediate layer by not less than 25.
In the above conventional golf balls, since the adhesion between the intermediate layer and cover is not sufficiently obtained, the adhesion is improved by roughing the surface of the intermediate layer, such as abrading the surface after forming the intermediate layer to form the cover thereon. However, the adhesion between the intermediate layer and cover is not sufficiently obtained yet by only the method, and it is problem that the durability is degraded because of the loss of the rebound characteristics and spin amount of the resulting golf ball.
In order to solve the problem, it has been attempted to interposing an adhesive layer between the intermediate layer and cover to improve the adhesion therebetween (Japanese Patent Kokai publication No. 179795/1998). The adhesion between the intermediate layer and cover is improved, but it is problem that a process of forming the cover on the intermediate layer is complicated to degrade the productivity.
Therefore, it is required to provide a golf ball having better rebound characteristics, better flight performance and better durability while maintaining good productivity.